Claims that global warming can be braked by dissolving huge quantities of rock in the sea to absorb carbon emissions are laden with flaws, a study published on Tuesday says.

The analysis is the latest scientific appraisal into geo-engineering, or techniques that are being promoted as quick fixes to the climate crisis.

Scientists in Germany probed the feasibility of "enhanced weathering," in which a plentiful magnesium-silicate rock called olivine would be crushed and scattered in the oceans.

Doing so would make the seas more alkaline and thus better able to absorb man-made carbon dioxide (CO2) emissions from the air.

The oceans are a massive carbon "sink," having absorbed roughly half of the fossil-fuel gases we have emitted, researchers say.

But this sponge-like ability is thought to be in worrying decline. As a result, comparatively more heat-trapping gases are entering the atmosphere, and the oceans themselves are becoming more acid, threatening many marine species.

Writing in the British journal Environment Research Letters, scientists at the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven calculated that if three billion tonnes of olivine were deposited into the oceans each year, it could compensate for only around nine per cent of today's carbon emissions.

And nearly a third of those emissions would indirectly return to the atmosphere because of the energy cost of grinding the rock to a diameter of around one micrometre -- about 10 times finer than a grain of talcum powder -- so that it could dissolve.

There could also be a shift in the fundamental marine ecosystem, as one species of phytoplankton, benefiting from the chemical change in the ocean, won out against another.

The findings are based on lab research into the CO2 absorption abilities of olivine, which were then factored into a powerful US model for ocean circulation.

"If this method of geo-engineering was deployed, we would need an industry the size of the present-day coal industry to obtain the necessary amounts of olivine. To distribute this, we estimate that 100 dedicated large ships with a commitment to distribute one gigatonne of olivine per year would be needed," lead author Peter Koehler said.

"Taking all our conclusions together -- mainly the energy costs of the processing line and the projected potential impact on marine biology -- we assess this approach as rather inefficient," said Koehler.

"It certainly is not a simple solution against the global warming problem."

Once dismissed as science fiction, geo-engineering has gained a respected audience in recent years as carbon emissions scale new peaks and UN talks on tackling climate change make meagre progress.

In parallel, though, scientists are giving closer scrutiny to what these once-scorned ideas might mean.

In the past year, studies delving into two other geo-engineering schemes have sounded caution about their cost effectiveness and potential impact on the environment.

The ideas comprise sowing the stratosphere with light sulphate particles to reflect the Sun's rays and thus cool the planet; and dumping iron particles in the ocean to stimulate plankton that suck up CO2 at the surface.